GaN (gallium nitride) is one of Group III-V compound semiconductors and has a wurtzite-type crystal structure belonging to a hexagonal system.
In recent years, a GaN substrate being a single crystal substrate configured by only a GaN crystal has attracted attention as a substrate for a nitride semiconductor device.
A nitride semiconductor is also called a “III nitride-based compound semiconductor”, “nitride-based Group III-V compound semiconductor”, “GaN-based semiconductor” or the like, and includes, besides GaN, compound(s) where gallium in GaN is partially or fully substituted with other Group 13 elements (B, Al, In, or the like) in the periodic table.
One particularly useful GaN substrate is a C-plane GaN substrate having a main surface parallel to or substantially parallel to the C-plane.
The C-plane GaN substrate has a gallium polar surface being a main surface on the [0001] side and a nitrogen polar surface being a main surface on the [000-1] side. The gallium polar surface is currently mainly used for formation of nitride semiconductor devices.
A GaN crystal grown by an HVPE (Hydride Vapor Phase Epitaxy) method is used for a commercially produced C-plane GaN substrate.
A seed used in the growth of a GaN crystal by an HVPE method is a GaN template obtained by growing a c-axis-oriented GaN film on a single crystal substrate different in composition from GaN, such as a sapphire substrate or a GaAs substrate, by an MOVPE (Metal Organic Vapor Phase Epitaxy) method. A GaN crystal is grown on the GaN template in the c-axis direction.
Examples of dopants used for imparting conductivity to a GaN crystal include oxygen (O). GaN is strongly facet-dependent with respect to the uptake of oxygen, and a sufficient amount of oxygen cannot be added to a GaN crystal in a growth mode where a growth surface is covered with C-plane facets. In the method described in JP-A-2002-373864 (Patent Document 1), pits are generated on the growth surface of GaN (the surface of a growing GaN crystal) growing in the c-axis direction, to expose facets other than the C-plane facets. The pits are generated by controlling growth conditions.
JP-A-2003-165799 (Patent Document 2) and JP-A-2006-66496 (Patent Document 3) disclose a method in which a dot pattern is formed on the surface of a GaN template with a growth mask in order to generate a surface pit accompanied by a core (“closed defect accumulating region”).
The core is an area having no continuity with the surrounding area in terms of a crystal structure, and the typical example is a domain where the polarity of a GaN crystal is locally inverted (an inversion domain). While the growth direction of the GaN crystal is the [0001] direction (+c direction) in areas other than the inversion domain, it is the [000-1] direction (−c direction) in the inversion domain.
In Patent Document 2, a polycrystalline area, and also a crystal area partitioned from the surrounding area with grain boundaries, plane defects, aggregates of line defects, or the like, are exemplified as the cores other than the inversion domain.